More narrowly, binders are liquid or dough-like substances that harden by a chemical or physical process and bind fibres, filler powder and other particles added into it.
Binders are loosely classified as organic (bitums, animal and plant glues, polymers) and inorganic (lime, cement, gypsum, liquid glass, etc.).
For example, in the compound WC-Co (Tungsten Carbide used in cutting tools) Co constitutes the binding agent for the WC particles.
Tensile strength is greatly improved in composite materials consisting of resin as the matrix and fiber as a reinforcement.
Glue is traditionally made by the boiling of hoofs, bones, or skin of animals and then mixing the hard gelatinous residue with water.
[1] Larger amounts of dry substance are added to liquid binders in order to cast or model sculptures and reliefs.
Traditionally straw and natural fibres are used to strengthen clay in wattle-and-daub construction and in the building material cob which would otherwise become brittle after drying.